In mobile communication systems such as a cellular phone, etc., third-generation CDMA (Code Division Multiple Access) systems are currently providing various types of services. Mobile communication systems of the next generation or later, which enable a faster communication, are currently under study. LTE (Long Term Evolution) is currently under study in 3GPP (3rd Generation Partnership Project). For example, in LTE, a method by which a mobile station applies intermittent reception (DRX: discontinuous reception) during not only a standby state but also a communication state. This aims at reducing power consumed by the mobile station because power increases with a speedup of a communication.
FIG. 1 is a schematic diagram illustrating the timings of intermittent transmission and reception of a mobile station. An upper portion illustrates the timing of a transmission from a base station to the mobile station, whereas a lower portion illustrates the timing of a transmission from the mobile station to the base station. As illustrated in FIG. 1, the mobile station periodically verifies whether or not a data transmission to the mobile station itself is made while applying the intermittent reception. Also for an upstream transmission, the mobile station periodically makes an intermittent transmission (DTX: discontinuous transmission) in order to maintain the synchronization of an intermittent communication.
In the meantime, in conventional technology, if it becomes necessary to switch a base station at a connection destination by performing a handover due to a move of the mobile station during the intermittent communication, the mobile station initially needs to change the communication state from the intermittent communication state to an active state before executing the handover process. If the mobile station determines to require a handover, it transmits a state transition request to change the communication state to the active state to the base station at the next intermittent transmission timing.
FIG. 2 illustrates data transmission/reception timings on the side of the mobile station when a handover is performed during an intermittent communication in the conventional technology. If the mobile station determines to require a handover on the basis of measurement results by periodically measuring a reception level such as a reception signal intensity, etc., it transmits the state transition request to the base station at the next transmission timing. The base station that receives the state transition request changes the communication state of the device, namely, the base station itself from the intermittent communication state to the active state, and transmits a message to instruct the mobile station to change from the intermittent communication state to the active state at the transmission timing of the base station. Upon completion of transition from the intermittent communication state to the active state in both of the base station and the mobile station, a handover is performed thereafter.
FIG. 3 is an operational sequence when a handover is performed at the time of an intermittent communication in the conventional technology. With the conventional technology, the state transition request to change to the active state needs to be transmitted at intermittent transmission timing. Also a state transition instruction from the base station that receives the state transition request needs to be transmitted to the mobile station at the intermittent transmission timing. Accordingly, a handover during an intermittent transmission/reception period has a problem of a longer delay time than that of the normal communication (active) state as illustrated in FIG. 3. Additionally, there is a problem such that the handover is not properly performed because it is not complete within a predetermined duration due to a prolonged time required until the transmission of the state transition request.
Other handover techniques include a technique (for example, Patent Document 1) for changing to a wireless communication establishment state on the basis of an RRC connection request if it becomes necessary to perform a handover at the time of an intermittent communication, a technique (for example, Patent Document 2) by which a terminal making an intermittent communication forms a paging group and a soft handover is taken into account in a series of sequence, a technique (for example, Patent Document 3) for performing a handover by using a paging channel in an asynchronous system, and a technique (for example, Patent Document 4) for performing a handover under the initiative of a base station.
An object of the present invention is to provide a technique for changing from an intermittent communication state to a normal communication state earlier in a mobile station and a base station, which need to perform a handover, etc.    Patent Document 1: Japanese Laid-open Patent Publication No. 2005-286807    Patent Document 2: Japanese Laid-open Patent Publication No. 2004-194015    Patent Document 3: Japanese Laid-open Patent Publication No. 2004-504783    Patent Document 4: Japanese Laid-open Patent Publication No. 2000-69523